Since a III-nitride semiconductor material having a large bandgap has characteristics of dielectric breakdown electric field having one higher order digit and high maximum electron velocity of two times or more, as compared to GaAs or Si, it is noted as a high power device material of microwave/milimetric wave bands. The HEMT using the material is expected as a high power transistor for next generation wireless communication. However, the materials has the problems of current reduction due to voltage stress (current collapse), gate leakage current, heat generation upon a high power operation and the like, which are causes of breakdown/output voltage decreases or instability. Hence, the most urgent necessity is to solve the above problems, thereby increasing the performance-reliability to an application level.
In the FET structure using a III-nitride semiconductor material, an AlGaN/GaN hetero structure has been typically used which uses piezo-charges by a strained AlGaN layer, so as to supply the carriers.
For example, Vitusevich et al. reported a FET structure in which the AlGaN/GaN layer structure is used to make, as a carrier, two-dimensional electron gas induced by a piezo effect of a strained AlGaN layer (Non-Patent Document 1: Applied Physics Letters vol. 80, No. 12, pp 2126).
FIG. 3 shows a sectional structure of the FET reported by Vitusevich et al. As shown in FIG. 3, on sapphire substrate 301 are deposited Al0.16Ga0.84N nucleation layer 302 having a thickness of 40 nm, GaN layer 303 having a thickness of 1.1 μm, and Al0.33Ga0.67N layer 304 having a thickness of 23 nm. Then, source electrode 305 and Ti/Al/Ti/Au as drain electrode 306 are deposited. Then, heat treatment is performed at 800° C. for 40 seconds. Furthermore, gate electrode 307 made of Ni/Au is formed and Si3N4 protection film 308 is finally formed. With the structure, electrons induced by the piezo effect of the strained AlGaN layer are accumulated at an interface of GaN layer 303 facing Al0.33Ga0.67N layer 304, so that two-dimensional electron gas (2DEG) is formed.    Non-Patent Document 1: Applied Physics Letters vol. 80, No. 12, pp 2126